1. Field of the Invention
The present invention relates generally to surgical instruments used during implantation of orthopedic joint replacement prostheses, and relates more particularly to a saw guide for guiding the resection of bony defects of the proximal tibia to accommodate a spacer used with a tibial component of a knee prosthesis.
2. Background Information
Total knee joint replacement surgery involves replacing the natural articulating surfaces of the femur and tibia with prosthetic components formed of biocompatible materials providing artificial articulating surfaces. The femoral component is often formed of a biocompatible metal such as titanium or a titanium alloy, or a cobalt-chrome alloy, with an articulating surface that is highly polished. The tibial component is often formed of a biocompatible metal baseplate and a smooth articulating surface attached to the metal baseplate that is formed of a material such as ultra-high molecular weight polyethylene that provides low friction and a low wear rate in combination with the metal articulating surface of the femoral component.
Each of the femoral and tibial components include, in addition to the mutually articulating surfaces, respective femur and tibia bone engaging surfaces. The bone engaging surfaces are often planar, or combinations of planar surfaces, and it is therefore necessary to resect the distal end of the femur and the proximal end of the tibia to provide complementary shaped planar bone surfaces for close fitting engagement with the planar bone engaging surfaces of the prosthetic components. In the case of the tibia, it is generally sufficient to provide a single, generally horizontal planar osteotomy of the proximal tibia to form a planar surface that mates with and engages a single planar inferior surface of the tibial base plate. In some cases, however, bony defects of the proximal tibia affect one of the lateral and medial condyles to a greater depth than the other. While it would be possible to perform a single planar osteotomy of the proximal tibia at a depth sufficient to resect the greater bony defect, this has the effect of resecting more bone than is desirable from the less affected side. It is therefore preferable to perform an initial planar osteotomy of the minimum depth necessary to resect the less affected side of the proximal tibia to provide good seating of the tibial base plate. A second generally horizontal osteotomy of the more affected side can be performed to resect that side of the proximal tibia to a greater depth to remove the bony defect. A spacer can then be inserted between the inferior surface of the tibial base plate and the deeper planar bone surface. A vertical sagittal osteotomy intersecting the first and second planar osteotomies will also be necessary to completely resect the bone on the more affected side.
Because of the desirability of providing good prosthesis-to-bone contact, it is necessary that the osteotomies referred to above be performed with relative precision. The vertical sagittal osteotomy should be located accurately in the medial-lateral direction to ensure alignment and close spacing between the inner edge of the tibial spacer and the vertical bone surface. The two horizontal planar osteotomies of the proximal tibia should lie in planes that are parallel to one another or otherwise conform to the relationship between the upper and lower surfaces of the spacer. In addition, the vertical spacing between the horizontal osteotomy planes should accurately conform to the thickness of the spacer. While the surgeon will have some latitude during surgery to select the thickness of the spacer to be used to correct for the bony defect that is being resected, for reasons of practicality the spacers will usually be available in only a few standard thicknesses. It would therefore be desirable to provide a tibial spacer saw guide that can accurately guide the saw blade during resection of the proximal tibia to ensure a good fit of the spacer.
One example of a tibial prosthesis with which the present invention can be used advantageously is the prosthesis described in U.S. Pat. No. 4,963,152, issued Oct. 16, 1990, and assigned to Intermedics Orthopedics, Inc.